The present application is based upon and claims priority to Japanese Patent Application No. Hei 11-026989, filed Feb. 4, 1999, the entire contents of which is hereby expressly incorporated by reference.
1. Field of the Invention
This invention relates to an engine decompression device, and more particularly to an improved decompression device that is suitable for marine engines.
2. Description of Related Art
In many engine applications, the operator may be required to manually start an internal combustion engine. This may be true whether or not the engine is also provided with an electrical or otherwise operated self-starting mechanism. For instance, it is frequently the practice in outboard motors, and particularly those of small displacement, to incorporate a mechanism whereby the engine may be manually started. This is normally done by a rope or recoil starter mechanism that is associated with a flywheel on the upper end of the crankshaft.
However, in order to achieve good engine performance, it is also the practice to use relatively high compression ratios. The use of such high compression ratios gives rise to a rather large force that must be overcome by the operator to effect manual starting. There have been, therefore, proposed types of decompression devices which effectively lower the compression ratio of the engine during the manual starting procedure. Preferably, such devices should be operative so as to be automatic in nature wherein the compression ratio is lowered only long enough to facilitate starting and not long enough to interfere with the running of the engine once starting has been accomplished. That is, the decompression device must be released promptly when engine is started and not work above a selected idle engine speed. One of the proposed devices has a construction in which a decompression actuator is mounted on a camshaft for pivotal movement about a pivot axis extending generally normal to an axis of the camshaft. The actuator has a cam section which may hold directly or indirectly an exhaust valve of the engine in an open position, and a weight section or sinker section which has a weight or sinker and may move with centrifugal force produced by rotation of the camshaft so as to release the cam section from holding the exhaust valve in the open position. An example of such a device is disclosed in U.S. Pat. No. 5,816,208.
Such a decompression device is, of course, applicable to an engine which powers a marine propulsion device provided in an outboard motor. However, some problems are caused by this particular use and a special structure of the engine for the outboard motor. That is, the engine for the outboard motor is often used under a trolling condition that drives an associated watercraft very slowly. Since a trolling speed almost equals to an idle speed of the particular engine and is quite slow, it is necessary to decrease an engine speed at which the decompression device is released to the speed that is lower than the trolling or idle speed. This is extremely difficult. If, however, the releasing speed is not stable under the trolling speed, an engine stall is quite likely to occur during the trolling operation.
Meanwhile, the camshaft of the engine for the outboard motor extends generally vertically and is driven by the crankshaft which also extends generally vertically. This particular construction consequently results in the sinker on the decompression device being significantly influenced by gravity and, therefore, adds another problem: the center of gravity in the decompression actuator must be determined by carefully selecting a proper weight for and a position of the sinker in consideration of the influence of gravity so that the decompression actuator will release at a slow rotational speed.
In addition, due to a relatively small and restricted space between an intake valve and an exhaust valve, the decompression actuator usually cannot be disposed therebetween in outboard motor engines. Thus, the actuator often is placed above an exhaust valve if the exhaust valve is disposed above the intake valve, or placed below an exhaust valve if the exhaust valve is disposed below the intake valve.
In the prior construction, the cam section is placed lower than the pivot axis of the actuator and primarily positioned close to the exhaust valve because of gravity. Accordingly, the releasing speed of the decompression actuator can be determined based only upon centrifugal force exerted on the cam section. That is, the sinker section is not always needed and thus merely provided to adjust the releasing speed minutely. It is, therefore, relatively easier to release the cam section in the aimed slow engine speed.
However, it is complicated and difficult to release the actuator properly in the latter construction, because the cam section is placed upper than the pivot axis of the actuator. This means that the cam section will depart from the exhaust valve unless the sinker section has much weight or the length from the pivot axis to the sinker is much longer. The fact is apparently inconsistent with the requirement that the sinker must move promptly with a relatively small centrifugal force to release the cam section in an aimed slow engine speed.
A need, therefore, exists for an improved decompression device that can be released at an exceedingly slow engine speed. A further need exists for the device in which weight and a position of a sinker are relatively easily selected.
In accordance with one aspect of the present invention, an internal combustion engine comprises a cylinder block defining at least one cylinder bore therein. A crankshaft is journaled for rotation relative to the cylinder block at one end of the cylinder bore and driven by a piston reciprocating in the cylinder bore. A cylinder head closes the other end of the cylinder bore and defines a combustion chamber with the piston and the cylinder bore. An intake passage communicates with the combustion chamber through a valve port. An intake valve is provided for regulating flow through the valve port. An exhaust passage extends from an exhaust port in the combustion chamber for discharging exhaust products from the combustion chamber. An exhaust valve is provided for regulating flow through the exhaust port. A camshaft rotates about a camshaft axis and is driven in timed relationship with the crankshaft to actuate at least the intake valve or the exhaust valve. A decompression device is provided to at least partially open the intake or exhaust valve at least during a portion of the compression stroke to reduce the compression ratio of the combustion chamber to ease manual starting of the engine. The decompression device includes an actuator mounted on the camshaft for pivotal movement about a pivot axis that extends generally normal to the camshaft axis. The actuator has a first section to hold the intake or exhaust valve at least partially open when the actuator is placed at an initial position. The actuator also has a second section disposed opposite the first section relative to the pivot axis.
The actuator is configured such that its center of gravity is located away from the pivot axis by a sufficient distance d so as to cause a rotational moment of the actuator to be greater than a minimum moment necessary for the actuator to return to the initial position under its own weight. The center of gravity of the actuator also is located relative to the pivot axis such a displacement angle xcex8g is generally less than a marginal angle xcex8m. The displacement angle xcex8g is defined between a line, which extends through the pivot axis and the center of gravity, and a datum line, which extends through the pivot axis and lies generally parallel to the camshaft axis. This displacement angle xcex8g represents the angle at which the center of gravity lies away from the datum line along an arc defined by distance d. The marginal angle xcex8m corresponds to a maximum angle at which the center of gravity can be moved away from the datum line along the arc defined to by distance d and still have the actuator release from the initial position at a preset minimum rotational speed of the camshaft.
In accordance with another aspect of the present invention, an engine comprising a camshaft for activating at least one intake or exhaust valve. The camshaft is rotatable about a camshaft axis. A decompression device comprises an actuator that is mounted on the camshaft for pivotal movement about a pivot axis which extends generally normal to the camshaft axis. The actuator includes a cam section, which holds the intake or exhaust valve in at least a partially open position, and a sinker section that moves with centrifugal force produced by rotation of the camshaft so as to release the cam section from holding the intake or exhaust valve at least partially open. The cam section is disposed on one side of the actuator relative to the pivot axis and the sinker section is disposed on an opposite side of the actuator relative to the pivot axis. The actuator is configured such that is center of gravity lies to the same side of the pivot axis as does the sinker section.
Further aspects, features and advantages of this invention will become apparent from the detailed description of the preferred embodiments which follow.